SOMERSET, Pa. — Former Pennsylvania Sen. Rick Santorum, once the No. 3 Republican in the Senate, said Monday he's seeking the GOP nomination for president, vowing he's "in it to win."

Santorum, a favorite among his party's social conservatives, chose to confirm his plans during an appearance Monday morning on ABC's "Good Morning America" at the balloon-draped site he chose in the western Pennsylvania coalfields for his formal announcement.

He has enjoyed strong support in the past from the anti-abortion rights bloc in the Republican Party, and he enters the race four days after former Massachusetts Gov. Mitt Romney formally declared his candidacy. Speculation remains strong that former Utah Gov. Jon Huntsman and Minnesota Rep. Michele Bachmann will jump into the race along with former House Speaker Newt Gingrich, former Minnesota Gov. Tim Pawlenty and businessman Herman Cain and, who already are in. Former Arkansas Gov. Mike Huckabee, Mississippi Gov. Haley Barbour, Gov. Mitch Daniels of Indiana and entrepreneur-entertainer Donald Trump have said they're not running. There also has been speculation that former New York Mayor Rudy Giuliani, who sought the nomination in 2008, is considering making another run.

In the network interview, Santorum accused President Barack Obama of having a weak foreign policy, saying he doesn't feel he has stood up sufficiently to Iran and asserting he has done too little to speak out against Syrian President Bashir Assad for the violence there.

Santorum, whose immigrant grandfather worked in the Pennsylvania coalfields, has already hired a small staff and has made frequent visits to early voting states New Hampshire, Iowa and South Carolina.

The blunt-talking conservative lacks the name recognition and fundraising organization of his better-known rivals, but the two-term senator's advisers are counting on social conservatives who have huge sway in some early nominating states and have yet to settle on a favorite candidate.

So far, those social conservatives are weighing already declared candidates such as Pawlenty and potential contenders Bachmann and former Alaska Gov. Sarah Palin.

Santorum, a lawyer by training, had been laying the groundwork for a presidential bid when he lost a bruising re-election bid to the Senate in 2006. His opposition to abortion, gay marriage and embryonic stem cell research makes him an appealing candidate for conservatives. But his sometimes abrasive style alienated voters in Democratic-leaning Pennsylvania, and they replaced him with Bob Casey, an anti-abortion Democrat.

Santorum's policy positions align with national conservatives who now are looking at many of the expected candidates with skepticism.

Romney's changes of heart on gay rights and abortion do little to help his second presidential effort. Gingrich is twice divorced. Huntsman, who worked for three Republican administrations, nonetheless accepted Obama's offer to be the U.S. ambassador to China.

Santorum, 53, has his own hurdles to overcome: He has been out of elective office since 2007 and lacks the robust fundraising or personal wealth of his likely rivals.

Earlier this year, he established a presidential exploratory committee to start raising money and joined the first — though ill-attended — Republican presidential debate in South Carolina. He is expected at next Monday's debate in New Hampshire, which is likely to include more of the expected field.